Abstract:

This study examines the spatial and temporal distribution and life
history of the "offshore" component of the Northeastern Pacific spiny dogfish
(Squalus acanthias) population. Distribution was examined in relation to
latitude, depth, the Columbia River plume and large-scale climate changes.
Fishery and survey data indicate that the Northeastern Pacific spiny dogfish
population has a broad range along the Washington and Oregon coasts, with
the highest abundance occurring off the northern Washington and central
Oregon coasts. Catches, however, were patchy throughout the 37-year period
of available survey data. In terms of depth, the greatest abundances of spiny
dogfish were captured in shallow waters (55-184 m). An examination of the
influence of the Columbia River plume using a generalized additive model
(GAM) indicated that the fish were influenced significantly by the salinity,
chlorophyll and surface temperature patterns associated with the plume,
preferring the oceanic zone to the plume zone. In contrast, there was no
indication that the catch-per-unit-effort of spiny dogfish was influenced by the
Pacific Decadal Oscillation (PDO) or the El-Niño (ENSO) over 24-year period
of the National Marine Fisheries (NMFS) triennial shelf groundfish survey
(1977-2001). The multi-cohort age structure of the population, due to the
spiny dogfish's late age at maturity and long lifespan, as well as the low
fecundity make it difficult to detect the loss of a single cohort or two when
examining abundance trends. Moreover, effects on abundance may be time-lagged,
especially if, as is likely, the youngest cohort(s) is the most vulnerable
life stage. Their absence would not become evident until they were large
enough to have been captured by the survey gear. Sensitive abundance data
and/or age-structured data would be needed to identify a pattern.
I quantified the age, maturation and fecundity of the spiny dogfish and
use these data to develop an age-structured matrix model to examine the
sensitivity of the population's growth rate to changes in mortality (i.e. fishing).
Female spiny dogfish in this population have an average age at 50% maturity
of 28 years and males 20 years. Average length at 50% maturity was 85.0 cm
for females and 71.5 cm for males. Female fecundity was extremely low,
averaging eight pups per clutch biennially and ranging from four to 14. The
number of pups increased with length, but not age. Using these data, a
deterministic, female-based model was developed. Asymptotic population
growth rate (λ) was determined to be very low, 1.01, and comparable to
population growth rates calculated for "coastal" populations. When fishing
mortality was incorporated into the model, the fishery was only sustainable
when exploitation was low and strict size limits enforced. When fecundity was
doubled, the population growth rate increased from 1% to 3% per year. This
33% increase in population growth was equivalent to the effect of a decrease
in the age at first maturity of three years. Given that responses to changes in
population demographics through shifts in fecundity or spawning frequency
are likely constrained, responses would then seem likely to involve changes in
age at maturity. These life history traits translate into a low potential
population growth and as a consequence high susceptibility to overfishing.
My findings indicate that the Northeastern Pacific spiny dogfish are
relatively slow growing, late to mature, and have low fecundity and a patchy
distribution, with large catches occurring off the northern Washington and
central Oregon coasts in shallow waters (55-184 m). These life history traits
and distributional patterns are useful in the development of population models
to predict responses to environmental fluctuations or increased mortality and
thus the management of the population. It is evident from my findings that
periodic monitoring is necessary to track possible catch declines off the
northern Washington and central Oregon coasts and prevent collapse of the
population should it be targeted by a fishery and overfishing occur.